A one-month old baby has been admitted to the pediatric unit with the diagnosis of pyloric stenosis. Which of the following assessments is the highest priority for the nurse to report to the baby's primary health-care provider?

A. Undigested emesis is common in pyloric stenosis and is not the most urgent finding. B. An apical heart rate of 156 bpm is elevated, but it is not the most urgent concern compared to dehydration and electrolyte imbalances. C. A serum potassium of 3.6 mEq/L is within normal limits, so this is not the most pressing concern for the baby at this time. D. A sunken fontanelle is a sign of dehydration, which is a significant and urgent concern in pyloric stenosis. Dehydration can lead to severe complications, so this requires immediate attention.